The increasing demand for portable devices is driving the development of small, rechargeable batteries with high energy density. Lithium-ion batteries are the system of choice owing to their higher specific energy (100-150 Wh/kg), higher specific power (150-250 W/kg) and longer lifespan (>1000 cycles) in comparison to other types of batteries. These advantages result from the battery's high voltage (2.4-4.6 V) and the high theoretical capacity density of the lithium carrier ion (3862 Ah/kg). The performance of a lithium-ion battery is heavily influenced by the intercalation materials used in its anode and cathode. (1) The intrinsic properties of the intercalation material determine the cell's voltage, capacity, and stability, which in turn determines the individual cycle-life and total lifetime of the battery.
Carbonaceous compounds (e.g., graphite and coke) have been widely used as anode materials in lithium-ion batteries because their electrochemical potentials are similar to that of lithium metal and because, unlike lithium metal anodes, they do not form dendrites. Most of the known lithium/transition metal oxides [LiMO2 (M: Co, Fe, Mn, Ni)] and nanotubes, for example carbon and TiO2 nanotubes, (2,3) have been studied for use as cathode materials.
Layered lithium nickel dioxide (LiNiO2) was one of the first lithium-metal oxides considered (4) as a cathode material because of its favorable specific capacity. However, it was found that the layered crystalline structure of delithiated LixNiO2 would collapse after the exothermic oxidation of the organic electrolyte. The collapse of the layered structure results in accumulation of lithium atoms on the electrode/electrolyte interface. The accumulated lithium atoms form dendrites, which penetrate the separators and create a short circuit inside the battery that can cause an explosion. On the other hand, the crystalline structure of delithiated lithium cobalt dioxide (LixCoO2) is more stable than that of LixNiO2. LiCoO2 is widely used in commercial lithium-ion batteries; however, the capacity of LixCoO2 (˜150 mAh/g) is smaller than that of LixNiO2 (˜250 mAh/g).
To improve the capacity and stability of intercalation materials, several routes have been investigated. For example, lithium-metal oxides have been doped with inert di-, tri- or tetravalent cationic elements (e.g. Ti and Mg). These elements substitute for Ni or Co and stabilize the layered structure of the intercalation materials. However, this LiM1-xTix/2Mgx/2O2 (M:Co or Ni) phase (5) is difficult to synthesize. Another approach employs chimie douce (soft chemistry) to synthesize the layered lithium manganese dioxide (LiMnO2) phase. However, the layered phase is structurally unstable and transitions to an unstable spinel, LixMn2O4, during use. (6) While electrochemically attractive, these materials exhibit limited cycling life and storage capacity.
Tuning the morphology or texture of lithium-intercalated materials to produce porous, high surface area composites presents an alternative strategy for improving electrode capacities and stabilities. For example, electrodes fabricated from mesoporous vanadium oxide (V2O5) were reported to have capacities up to 100% greater than electrodes of polycrystalline non-porous V2O5 powder. (7)
Still, it is desirable to have an electrode material for lithium ion batteries that exhibits improved capacity and stability.